1. Field of the Invention
The present invention relates to an alerting cadence control system for use with a telephone network comprising a plurality of telephone sets.
2. Description of the Prior Art
The term "alerting cadence," as defined and used by U.S. industries involved with communication technology, refers to the cycle of ringing cadence of telephone sets (i.e., ringing cycle of subscriber telephone sets). Recent years have seen an appreciable increase in the demand for telephone sets with newly added functions following the implementation of various business communication services and ISDN (integrated services digital network) applications requiring such functions. Existing telephone sets are being replaced with or supplemented by these novel telephone sets. With more and more subscribers signing up for newly introduced services that rely on the new telephone functions, there is a growing need for a control system that flexibly controls the subscriber alerting cadence of the telephone network with efficiency.
A typical prior art alerting cadence control system will now be described with reference to FIG. 1. Subscriber transmission equipment 1 is connected on the one hand to a telephone exchange by a telephone line 2. Generally the subscriber transmission equipment 1 is located far away from the telephone exchange. The subscriber transmission equipment 1 is connected on the other hand to subscriber telephone sets 3 by subscriber lines 4, the telephone sets 3 being located within a certain area centering on the equipment 1.
Positioned where it is, the subscriber transmission equipment 1 comprises a subscriber interface card 5 connected to each subscriber telephone set 3 by a subscriber line 4 and a ringer generator 6 for outputting a high frequency signal S1. The subscriber interface card 5 includes a changeover switch 7 and an alerting cadence controller 8 that controls the switching action of the changeover switch 7 for control over the subscriber alerting cadence.
The changeover switch 7, comprising a movable contact 9 and terminals 10 and 11, enters one of three states when activated: one in which the movable contact 9 comes into contact with the terminal 10, another in which the movable contact 9 contacts the terminal 11, and another in which the movable contact 9 contacts neither of the terminals 10 and 11. The alerting cadence controller 8 is composed either of hardware containing logic circuits that realize the logic for controlling the switching action of the changeover switch 7, or of software made of programs in a ROM or the like for controlling the same action.
In the above setup, the alerting cadence controller 8 provides control over bringing or not bringing the movable contact 9 of the changeover switch 7 into contact with the terminal 10. This action or inaction places onto the subscriber line 4 or withholds therefrom the high frequency signal S1 coming from the ringer generator 6 for the control of the subscriber alerting cadence.
The above-described alerting cadence control system has among others a major disadvantage. That is, when a new subscriber service is offered to a new function-equipped telephone set replacing the old telephone 3, the hitherto-used subscriber interface card 5 must be replaced with a new subscriber interface card. In that case, the service engineer in charge of the replacement must travel all the way to the distant subscriber transmission equipment 1 for work. This means not only the inability to address the subscriber's need at short notice but also more man-hours, hence more costs, required to complete the task.